A Chinese man stands alone to block a line of tanks heading east on Beijing's Cangan Boulevard in Tiananmen Square on June 5, 1989. The man, calling for an end to the recent violence and bloodshed against pro-democracy demonstrators, was pulled away by bystanders, and the tanks continued on their way. The Chinese government crushed a student-led demonstration for democratic reform and against government corruption, killing hundreds, or perhaps thousands of demonstrators in the strongest anti-government protest since the 1949 revolution. Ironically, the name Tiananmen means "Gate of Heavenly Peace". (AP Photo/Jeff Widener)

House Speaker John Boehner, House Democratic Leader Nancy Pelosi and other members of Congress joined with Chinese human rights activists on Thursday to commemorate the 1989 Tiananmen Square massacre, which occurred 25 years ago on June 4.

The response by U.S. leaders comes as the Chinese government cracks down on political dissidents in advance of the anniversary.

More than 50 dissidents have been detained by the Chinese government in the past two months, a disheartening development for democracy activists who hoped President Xi Jinping would allow free discussion and demonstration as part of his socialist reform campaign, the "Chinese Dream."

“The 25th Tiananmen anniversary was a critical test for President Xi's claims to be delivering greater openness. But Xi has opted for repression over reform,” Amnesty International Secretary General Salil Shetty said.

The South China Morning Post reports that two more civil rights activists, Yu Shiwen and Chen Wei of Henan Province, were detained this week for "gathering a crowd to disturb public order." In April 2013, Yu and Chen organized a gathering to commemorate the 1989 massacre. It was the first such memorial held within China since the massacre, as the Chinese government forbids discussion of the event.

The remembrance ceremony in Washington featured several Chinese activists, including Chai Ling, who was present at the Tiananmen protests. Chai spoke against continuing human rights abuses in China. The advocacy group she runs, All Girls Allowed, compiles incidents of forced and sex-selective abortion as a result of China's one-child policy, which has strictly regulated reproduction of Han Chinese since 1979.

The one-child policy, which has decimated the sex ratio in China, was relaxed for qualified couples in late 2013. However, Critics argue the policy change was the result of demographic necessity rather than any new-found respect for human rights on the part of the Chinese government.

The Washington ceremony was led by Rep. Chris Smith, R-N.J., who chairs the Congressional-Executive Commission on China. On Wednesday, Smith introduced legislation to mark the anniversary and honor "the extraordinary sacrifice endured by thousands of peaceful Chinese democracy activists." The bipartisan resolution passed 379-1.